1. Field of the Invention
The present invention relates to electrical signal processing, and, in particular, to AC coupling networks.
2. Description of the Related Art
An AC coupling network is used to connect two sets of electrical circuitry, where the AC coupling network substantially blocks any DC component in the output signal from the first set of electrical circuitry from passing to the second set of electrical circuitry. In many electrical signal processing applications, it is also desirable for the AC coupling network to apply a variable amount of attenuation to the output signal received from the first set of electrical circuitry before it is passed to the second set, while maintaining a constant pole frequency for the network. Prior-art techniques for creating an AC coupling network with variable attenuation and constant pole frequency employ an active operational amplifier or buffer stage.
FIG. 1 shows a schematic circuit diagram of a prior-art AC coupling network 100 that employs an active operational amplifier Al having variable capacitors C1 connected at its inputs. In network 100, attenuation is controlled by adjusting the ratio of C2 to C1, while the AC coupling pole frequency ωAPCL is given by Equation (1) as follows:
                              ω          ACPL                =                  1                      R1            *            C2                                              (        1        )            By varying C1 and keeping C2 constant, the attenuation (or gain) of network 100 can be adjusted without affecting the AC coupling pole frequency.
FIG. 2 shows a schematic circuit diagram of a prior-art AC coupling network 200 that employs active buffer stages A2. In network 200, attenuation is controlled by adjusting the ratio of R4 to R3, while the AC coupling pole frequency ωAPCL is given by Equation (2) as follows:
                              ω          ACPL                =                  1                      R2            *            C3                                              (        2        )            By varying R4 and keeping R3 constant, the attenuation of network 200 can be adjusted without affecting the AC coupling pole frequency.
Both of these prior-art circuits can provide independent adjustment of attenuation and the AC coupling pole frequency. However, both circuits require active devices in the form of an operational amplifier or a buffer.